In a chemochromic sensor, a test material is allowed to react with a chemochromic reagent in a test area. A catalyst can be used to facilitate a reversible reaction. The reaction changes the optical property of the reagent, which can be measured with a light sensor.
Prior art chemochromic sensors typically use one-way light measurements of the test area. Light is generated by a light-emitting diode (LED) and sensed by a phototransistor, see, e.g., Benson et al., “Low-cost fiber-optic chemochromic hydrogen gas detector,” Proceedings 1999 U.S. DOE Hydrogen Program review, NREL/CP-570-26938, 1999.
Because the prior art uses one-way light measurements, calibration of the sensor with known quantities of the test material is necessary, and aging characteristics of the chemochromic reagent must also be known. Therefore, it is desired to provide a chemochromic sensor that uses nulling and differential measurements.